ABC News(LAS VEGAS) -- Authorities have revised the timeline for last week’s mass shooting in Las Vegas, raising new questions about the hotel’s response to an attack that killed 58 people and injured several hundred more.

The sheriff’s office initially said that Stephen Paddock’s rampage was brought to an end when security guard Jesus Campos arrived on the 32nd floor to check on a door alarm to discover the shooting in progress.

Federal agents told ABC News, however, the FBI has known for several days that the Sheriff Joseph Lombardo, chief of the Las Vegas Metropolitan Police Department, was providing inaccurate information to the public about the sequence of events inside the Mandalay Bay hotel.

Lombardo acknowledged on Monday, under pressure from the FBI, that Campos reported that he had been shot at least six minutes before the rampage began.

“What we have learned is Mr. Campos was encountered by the suspect prior to his shooting to the outside world,” Lombardo said in a press conference.

According to Brad Garrett, a former FBI agent and ABC News consultant, the change is significant. If the hotel knew there was an active shooter on the 32nd floor six minutes before the rampage began, Garrett says, it raises the question of whether they could have done more to stop him.

“And those six minutes, you may not have been able to get the police there, but you may have been able to create a disruption of some sort that would have kept him from doing what he ultimately did,” Garrett said.

Officials briefed on the investigation have also told ABC News that Paddock used the hotel’s service elevator in the days leading up to the attack. Access to the service elevator, sources say, was a perk for high-rollers.

On Tuesday, deputy sheriff Tom Roberts told ABC’s local affiliate station KTNV-TV the hotel did the best it could.

“You have that human factor where it goes to a person and they end up loggin’ into a computer, then they make a call to dispatch – at the same time he may have already started firing outside,” Roberts said.

The owner of the Mandalay Hotel, MGM, would not answer questions from ABC News.